(a) Field of the Invention
The present invention relates to a power factor correction circuit and a driving method thereof.
(b) Description of the Related Art
A power factor correction circuit controls a switching operation of a power switch in order to generate an average input current that equals a full-wave rectified voltage (hereinafter referred to as an input voltage) in shape. A bridge diode of the power factor correction circuit generates an input voltage by rectifying AC input power, and in this case, the input voltage is a full-wave rectified sine wave.
FIG. 1 shows an input voltage, an inductor current, and an average input current of a conventional power factor correction circuit.
As shown in FIG. 1, an inductor current of the power factor correction circuit is iteratively increased and decreased according to a switching operation of the power switch, and a peak value of the inductor current depends on a full-wave rectified sine wave. Then, the average input current and the full-wave rectified voltage are the same in waveform.
The power factor correction circuit sets the maximum current to control the switching operation of a switch within a range that the inductor current does not exceed the maximum current. The reason that the power converter sets the maximum current for protection and the reason that the power factor correction circuit sets the maximum current are different from each other. Since a switch current of the power factor correction circuit depends on an arbitrary input voltage, the peak value of the switch current is decreased when the input voltage starts to decrease after the peak value of the switch current reaches the maximum current. Therefore, it is difficult to control a protection of the power factor correction circuit to use the maximum current. Furthermore, when the input voltage is low and in the overload state, users prefer that the switch current reaches the maximum current in a ridge period of an input voltage waveform.
The power factor correction circuit senses a time point when the inductor current becomes zero using an auxiliary coil coupled to the inductor. The power factor correction circuit turns on the switch when the inductor current is zero. When a zero current sense signal telling the inductor current to become zero is not generated for a predetermined time, the power factor correction circuit turns on the switch with a predetermined cycle by force. This operation occurs at the start-up state or when the input voltage is close to zero. However, the auxiliary coil may be short-circuited in the power factor correction circuit. Then, inductance of a magnetically combined inductor of the power factor correction circuit is decreased due to the short circuit of the auxiliary coil. Since the auxiliary coil is short-circuited and no signal is generated therein, sensing a time point when the inductor current becomes zero fails, and a current flowing in the inductor is suddenly increased due to an inductance decrease of the power factor correction circuit.
When the zero current detection signal is not generated, the switch is turned on by force with the predetermined cycle so that the switch current reaches the maximum current within a very short period of time. In this case, the short circuit of the auxiliary coil is continued for a long period of time, temperature of a transformer formed of the inductor and the auxiliary coil is increased, and a coating of the auxiliary coil may be melted due to the increased temperature. Moreover, when the auxiliary coil is continuously short-circuited, heat or explosion due to an overcurrent flowing to the auxiliary coil, the inductor, and the power switch may damage a human body. Therefore, the short circuit of the auxiliary coil needs to be sensed.
However, as previously stated, the auxiliary coil may not be short-circuited even though the inductor current is higher than the maximum current due to the inductor current characteristic of the power factor correction circuit. In the power factor correction circuit, it is difficult to determine whether the overcurrent is caused due to a short circuit of the auxiliary coil.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.